FIG. 1 shows a conventional output circuit arranged in an integrated circuit. FIG. 2 is a Figure showing the characteristics of transistors Q1 and Q2 of the integrated circuit. An input signal S generated in a separate integrated circuit is supplied via a buffer B1 to a base of NPN transistor Q1. A collector of the transistor Q1 is connected via a resistance R2 to a power voltage source. An emitter of the transistor Q1 is connected via a resistance R1 to a ground GND, and also connected to a base of NPN transistor Q2. An emitter of the transistor Q2 is connected to the ground GND, and a collector of the transistor Q2 is connected to an output terminal, to which an external load is connected.
In this open collector circuit, the transistor Q2 is designed to operate in a saturation range as shown in FIG. 2. FIG. 3(a) is a timing chart of V.sub.S (voltage of input signal S), V.sub.Q1B (base voltage of transistor Q1), I.sub.Q1E (emitter current of transistor Q1), V.sub.0 (voltage of the output terminal) and I.sub.Q2C (collector current of transistor Q2). FIG. 3(b) is an enlarged Figure of a graph of current versus time for a part of I.sub.Q2C. This has been obtained by simulation. As seen in FIGS. 3(a) and 3(b), I.sub.Q2C has a maximum value when V.sub.S rises. The reason for this maximum is described below.
An initial status of the transistor Q2 is in an active range, and until the transistor transitions from the active range to a saturation range, a current, that is a product of the base current of the transistor Q1 and the square of H.sub.fe (current amplification factor of transistors Q1 and Q2), runs through the transistor Q2.
Accordingly, when input signal S transitions from a high level to a low level and the voltage level of the output terminal rises, since the transistor Q2 is in the active range, an overcurrent of h.sub.fe times the base current of the transistor Q2 runs through the transistor Q2. This overcurrent causes the voltage level of the ground to float up, generating noise, which is a problem in the prior art.